Implantable medical electrical stimulation and/or sensing leads intended for chronic implantation in the body are well known in the fields of cardiac stimulation and monitoring, including cardiac pacing and cardioversion/defibrillation, and in other fields of electrical stimulation or monitoring of electrical signals or other physiologic parameters. In the field of cardiac stimulation and monitoring, endocardial leads are placed through a transvenous route to locate one or more sensing and/or stimulation electrode along or at the distal end of the lead in a desired location in a chamber of the heart or a blood vessel of the heart. In order to achieve reliable sensing of the cardiac electrogram and/or to apply stimulation that effectively paces or cardioverts the heart chamber, it is necessary to accurately position the electrode surface against the endocardium or within the myocardium at the desired site and fix it during an acute post-operative phase until fibrous tissue growth occurs.
The pacemaker or implantable cardioverter/defibrillator (ICD) implantable pulse generator (IPG) or the monitor is typically coupled to the heart through one or more of such endocardial leads having a lead body extending between a proximal lead connector assembly and distal electrode. The proximal lead connector assembly comprising one or more connector element is connected with a connector header of the IPG or monitor. The lead body typically comprises one or more insulated conductive wire surrounded by an insulating outer sleeve. Each conductive wire couples a proximal lead connector element with a distal stimulation and/or sensing electrode. An endocardial cardiac lead having a single stimulation and/or sensing electrode at the lead distal end and a single conductive wire is referred to as a unipolar lead. An endocardial cardiac lead having two or more stimulation and/or sensing electrodes at the lead distal end and two or more conductive wires is referred to as a bipolar lead or a multi-polar lead, respectively.
In order to implant an endocardial lead within a heart chamber, a transvenous approach is utilized wherein the lead is inserted into and passed through the sub-clavian, jugular, or cephalic vein and through the superior vena cava into the right atrium or ventricle as depicted, for example, in U.S. Pat. No. 5,545,203. An active or passive fixation mechanism is incorporated into the distal end of the endocardial lead and deployed to maintain the distal end electrode in contact with the endocardium or within the myocardium at the implantation site. An introduction mechanism, e.g., a stiffening stylet and/or a guide catheter, is employed to advance the distal electrode(s) to the electrode implantation site(s).
More recently, endocardial pacing and cardioversion/defibrillation leads have been developed that are adapted to be advanced using particular guide mechanisms into the coronary sinus and coronary veins branching therefrom in order to locate the distal electrode(s) adjacent to the left ventricle or the left atrium. The distal end of such coronary sinus leads is advanced through the superior vena cava, the right atrium, the valve of the coronary sinus, the coronary sinus, and into a coronary vein communicating with the coronary sinus, such as the great vein. Typically, coronary sinus leads do not employ any fixation mechanism and instead rely on the close confinement within these vessels to maintain each electrode at a desired site although active fixation mechanisms, e.g., minute helixes that are screwed into the vessel wall, can be employed.
The heart beats approximately 100,000 times per day or over 30 million times a year, and each beat stresses the lead conductors and insulation. Over the years of implantation, the lead conductors and insulation are subjected to cumulative mechanical stresses as well as material reactions that can result in degradation of the insulation or fractures of the lead conductors with untoward effects on IMD performance and patient well-being. The endocardial lead bodies of pacing and ICD leads are subjected to continuous stretching and flexing as the heart contracts and relaxes and are formed to be highly flexible, resilient, and durable employing durable bio-compatible lead conductor and insulator materials and structures.
Initially developed chronically implanted unipolar and bipolar cardiac pacing leads employed flexible silicone rubber tube having a single lumen and two lumens, respectively in which single filar wire conductors formed of stainless steel and later of MP35N alloy that were wound into wire coils were inserted and electrically coupled to a proximal lead connector element and a distal pace/sense electrode. Early implantable, endocardial and epicardial, bipolar cardiac pacing leads of the type disclosed in U.S. Pat. No. 3,348,548, that were clinically implanted in the 1960s, had two lumens arranged side-by-side and coiled wire conductors disposed in each lumen. The wire coil and silicone rubber tube of such endocardial pacing leads allow the lead body to stretch axially and provide a coil lumen for receiving a stiffening stylet during transvenous lead advancement.
Such lead bodies fabricated at that time were relatively large in diameter, and the side-by-side arrangement was believed to be responsible for lead body fractures. In addition, effective passive and active distal fixation mechanisms were not available, and displacement of the distal electrodes were common occurrences. Surgeons resorted to stiffening the lead body by leaving the stylet in place to prevent dislodgement, but the stiffening stylet then often fractured within the wire coil lumen, and the sharp broken stylet ends initiated a lead fracture. Moreover, a portion of the lead body was (and is to the present time) often implanted between the first rib and right clavicle as illustrated in the above-referenced '545 patent, and the stresses on the lead body caused the relatively large diameter lead insulator and body to be crushed and fracture.
A great deal of effort has been undertaken over the years to reduce these complications by developing fracture and crush resistant lead bodies and to provide the above-mentioned active and passive fixation mechanisms that effectively reduced lead dislodgement. At the same time, many other material and structural improvements have been made in lead conductors, lead insulators, electrodes, and various other electrical medical lead components to reduce the lead body diameter and increase its lubricity, to increase conductivity of the lead conductor, to increase the number of lead-borne electrodes, to decrease stimulation thresholds, particularly cardioversion/defibrillation and pacing thresholds, and optimize sensing of the electrogram, to reduce inflammation at the electrode-tissue interface, to optimize electrode shapes and materials, to incorporate sensors in some instances, and to otherwise simplify implantation, reduce complications, assure reliable pacing and cardioversion/defibrillation, and increase the implantation lifetime of such cardiac leads and other electrical medical leads. In particular, many changes have been made in the materials used in and the fabrication of lead bodies extending between the proximal lead connector assembly and distal electrodes(s) and sensor(s).
Most current endocardial cardiac leads employ multi-filar, parallel-wound, coiled wire conductors electrically connected in common in an electrically redundant fashion as a single polarity lead conductor in each of the unipolar, bipolar and multi-polar lead configurations. Such redundant coiled wire conductors of bipolar and multi-polar lead bodies are coaxially arranged about the stiffening stylet receiving lumen and insulated from one another by coaxially arranged insulating sheaths separating each coiled wire conductor from the adjacent coiled wire conductor(s). The number of separate lead conductors that can be incorporated in a lead body of a given diameter is limited in this coaxial winding approach.
In certain cases, the need for increased numbers of lead conductors in the lead body has led to the development of separately insulated, coiled wire conductors that are parallel-wound with a common diameter and are separately coupled between a proximal connector element and to a distal electrode or terminal in the manner described in commonly assigned U.S. Pat. No. 5,007,435, for example. The coaxial construction technique may also be combined with the parallel-winding technique to multiply the total number of separate coiled wire conductors accommodated within a specified endocardial lead body outer diameter.
Improvements in stranded wire conductors and lead body materials have more recently led to the combination of substantially straight, stranded wire conductors and at least one coiled wire conductor providing a stylet lumen as illustrated in commonly assigned U.S. Pat. Nos. 5,584,873, 6,052,625, and 6,285,910, for example. In these lead bodies, ETFE sleeve insulators encase the stranded wire conductors, and a PTFE sleeve insulator surrounds the multi-filar, coiled wire, conductor. The insulated wire conductors are received in lumens of an elongated lead body insulator that also incorporates elongated, empty, compression lumens.
Typically, the lumens in lead body insulators receiving substantially straight, stranded wire conductors or coiled wire conductors are not otherwise filled with a filler or the like. However, the above-referenced '203 patent discloses coaxial lead bodies of the types disclosed above as well as a lead body incorporating multiple, coiled wire conductors arranged side-by-side within lumens of the lead body insulator. The lumens are also reinforced against crushing with a liquid polymer, e.g., Silastic® silicone rubber, silicone rubber adhesive or polyurethane, that solidifies in place. However, encasing the turns of the coiled wire conductors along the length of the lead body with such materials can stiffen the lead body unduly and may increase the likelihood of stress-related fracture at other points along the lead body.
To some extent, it has been recognized that the relative movement of lead conductors with respect to the surrounding wall of silicone rubber tube can abrade the conductors or tube, perhaps due to the abrasive action of silica of the silicone rubber compound, and cause the lead body to fail. In commonly assigned U.S. Pat. No. 5,796,044, coiled wire, single filar and multi-filar, conductors are disclosed that are sheathed loosely within a separate, coiled, insulating sheath allowing a gap or space to be present between the exterior surface of the coiled wire conductor and the adjacent interior surface of the insulating sheath. The insulating sheath is loosely fitted around the coiled wire conductor to avoid concentrating corrosion effects at the site of a defect, allowing any corrosion that may occur as a result of the defect to be spread over a larger wire surface. The coiled insulating sheath is preferably included within the lumen of a non-coiled outer insulating sheath.
It is suggested in U.S. Pat. No. 3,333,045 that the tube lumen be backfilled with a liquid silicone fluid or powdered ETFE to lubricate the surfaces of the lumens of silicone rubber tube receiving stranded, drawn-brazed stranded (DBS) wires, loosely coiled into coiled wire conductors. Incorporating such non-conductive lubricating or reinforcing materials within the conductor lumen of a lead body insulator may or may not reduce the possibility of lead conductor fracture through abrasion or crushing. The electrical connection between the distal electrode or sensor and the proximal lead connector element is interrupted if a fracture of the lead conductor does occur with or without the lubricating or reinforcing materials within the lead conductor lumen.
It has been proposed to include other materials or mechanisms to provide a form of redundancy with the coiled or straight stranded wire conductor to compensate for a complete fracture or the reduced conductivity attendant to a partial fracture of the conductor. In U.S. Pat. No. 4,033,355, the coiled wire conductor is tightly fitted within a conductive silicone rubber tube, e.g., silicone rubber compounded with conductive particles, e.g., carbon. The tight fitting is determined to be necessary to ensure that the wire coil turns make intimate contact with the conductive silicone rubber so that a section of conductive silicone rubber bridges any fractured ends of the wire conductor. An internal block of conductive silicone rubber surrounding a section of the coiled wire conductor within the lead connector assembly of the cardiac pacing lead disclosed in U.S. Pat. No. 3,924,639 is provided to make a temporary electrical connection with the lead conductor. The use of conductive silicone rubber as disclosed in the '355 patent raises the same issues of reduced lead body flexibility and abrasion possibly increasing the risk of fracture as the use of the tight fitting electrically insulating silicone rubber disclosed in the above-referenced '203 patent. While such silicone rubber materials can be made conductive by incorporating suspended conductive particles to a certain degree, but the conductivity does not match that of the lead conductor itself.
Endocardial leads that have an increased resistance to fracture and the capability of continued function after fracture of a lead conductor are disclosed in commonly assigned U.S. Pat. Nos. 6,018,683, 6,061,598, 6,119,042 and 6,285,910 and in the above-referenced '044 patent. The endocardial leads are provided with a monofilar or multi-filar coiled wire conductor that extends along the length of the lead body between a proximal electrical connector element and a distal electrode in a conventional manner. In addition, a stranded wire conductor extends loosely along the coiled wire conductor from a point along the lead body located proximal to the point of expected breakage of the coiled wire conductor to a point along the lead body located distal to the point of expected breakage. In certain embodiments, the proximal and distal ends of the stranded wire conductor are electrically and mechanically coupled to the coiled wire conductor, limiting the extensibility of the coiled wire conductor, rendering the coiled wire conductor less susceptible to axially applied tensile forces, and also providing for continued electrical connection between the connector element and the electrode in the event that the coiled wire conductor fractures intermediate the proximal and distal ends of the stranded wire conductor. In other embodiments, the stranded wire conductor is coupled only at its proximal or distal end to the coiled wire conductor or may simply be located in the same lumen as the coiled wire conductor without mechanical connection to the coiled conductor. Due to the confines of the lumen, the stranded and coiled wire conductors come into contact at numerous points along their respective lengths so that a mechanical connection is not necessary.
The fabrication of electrical medical lead bodies often requires directing lead conductors through cavities in the lead body insulator other than conductor lumens per se in order to make electrical connections to lead connector elements or distal electrodes or sensors. For example, the lead body insulator of the endocardial leads disclosed in the above referenced '625, '873, '683, '598, '042 and '910 incorporate bifurcation sleeves or trifurcation sleeves in order to connect two or three, respectively, connector assemblies to selected lead conductors that are diverted through branch sleeve cavities or lumens. These sleeve lumens are typically backfilled with liquid silicone rubber adhesive that solidifies within the cavity about the short segments of conductor wire traversing the cavity and immobilizes them. At times, the immobilization of the lead conductor segment can cause or does not prevent the lead conductor traversing the sleeve lumen to fracture due to chronically applied stress.
It is therefore desirable to provide a relatively simple, electrically redundant, bridging of a fractured wire conductor traversing a cavity, lumen or other space of the lead body.